Solvent extraction is an efficient method for selectively removing, purifying and concentrating certain elements or compounds from a solution. Two characteristics of the solvent extraction process make it a particularly efficient method of separation: (1) solutes can be selectively removed, and (2) they can be concentrated from a very dilute stream. In some cases, the main purpose is to recover values from mixed solutions, and in other cases, the major purpose is to remove undesired components, including pollutants, from liquid process streams.
There are two types of solvent extraction systems in common use, the mixer-settler system and the pulse or spray column system. The mixer-settler comprises a mixer for each stage for mixing or agitating the phases together to contact the phases. After the mixer, the mixed phases flow into a settler, wherein the phases separate. Mixer-settler units can be operated with a very high degree of contact between the two phases, but suffer from considerable complexity of equipment and operation due to the requirements of impellers for stirring, interstage pumping, and piping. They also require large floor space. They are also difficult to clean and maintain, especially for processing liquids that may be corrosive or particle laden.
Pulse or spray columns are devices where light and heavy phases move counter-currently through a vertical vessel. Pulse and spray columns are simple and without moving parts, but the throughput rates are low and they suffer from severe backmixing, which greatly reduces the extraction efficiency.